Electrical apparatus



Jan. 1 H. BEDFORD ETAL 2,538,438

ELECTRICAL APPARATUS Original Filed Feb. 25, 1944 E L I Q 5 A 3 0 l7 1%M MWHAM/ I 2, and e.

Patented Jan. 16, 1951 2,538,48, ELECTRICAL APPARATUS I Leslie HerbertBedford ahajArumr Henry Ashford Wynn, London, England,' fl assignors.to"A. C.

company ,Cossor, Limited,. ,I ondn; 'Englandya Britishff Originalapplication w s; 25,1944, Serial No. 523,890., Dividedand thisapplication February ,9, 1949, Serial No. 75,412;

1 This invention relates to tuning systems-for resonant electriccircuits, and is particularly applicable to radioequip'm'ent'intendedfor operation on frequencies of the order of megacycles or tens ofmegacycles. y p

The principal" object of the present invention is to provide an improvedtuning system whereby the resonance frequency-of a variable frequencycircuit canrea'dily be changed by a fixed amount which is constantirrespective of the frequency to which the circuit is tuned before thechange is made.

A-further object is to provide a tuning system wherein this frequencychange can be made by means of a simple switch and condenserarrangement.

One embodiment of the present invention will now be described, by way ofexample, with reference to the accompanying drawing which is a schematiccircuit diagram of the embodiment.

The circuit shown may be required, for example, in high frequency radioequipment which serves alternately as a transmitter and as asuperheterodyne receiver, both operating on the same signal frequency,and wherein the same oscillator is used for sending and receiving. Insuch an equipment, the resonant frequency must be changed, byan amountequal to the relatively small intermediate frequency, when switchingover from sending to receiving. In the circuit shown, this result isachieved by switching an additional condenser into or out of circuit.

In the, drawing an inductance L is tuned by the resultant capacity of anetwork of condensers I, Condensers i and 2 are connected in separatepaths across inductance L, and the frequency of the circuit is varied bysimultaneous variation of condensers I and 2 by suitable ganging meansrepresented at 4. Fixed condenser 3 is connected in series with variablecondenser 2 and may be .short-circuited by switch S. The resonantfrequency is decreased, by an amount independent of the setting ofganged variable condensers i and 2, by closing switch S andtherebyeliminating fixed condenser 3 from the circuit. In order to obtain thisindependence it is necessary that the capacity variations of condenser 2shall be determined throughout its range of variation according to aspecial law. This law may most simply be expressed by stating thecapacity of condenser 2 in terms of invariable quantities and of theresonant frequency. Thus stated the law is as follows:

3Claims. (oust-40) In Great, Britainv C=' l; r 1 f (f C2 capacitance ofcondenser 2,

C3 capacitance of condenser 3,

,; =resonant frequency, that is to say the fre- F fixed frequency changeefiected'by closing switch S.

The vane shapes for condenser 2 will, of course, depend upon the vaneshapes of condenser I. If condenser I has a straight-line-capacity lawand the maximum values of condensers I and 2 are fairly nearly equal,the vane shapes for condenser 2 are particularly convenient, whilecondenser 3 may be suificiently large to swamp 1944, the subject matterof which is being claimed in continuation application, Serial No.87,268, filed April 13, 1949.

We claim:

1. A resonant circuit comprising an inductor,

a variable capacitor connected in parallel with said inductor whosecapacitance changes with movement of a movable member thereof inaccordance with a first law, a series arrangement of a fixed capacitorand a variable capacitor also connected in parallel with said inductor,means providing mechanical ganging between said variable capacitors, anda switch device connected to short-circuit said fixed capacitor andthereby to change the resonance frequency of said circuit, said variablecapacitor in said series arrangement having capacity determiningstructure distributed over the range of movement of said ganging meansaccording to a second law differing from said first law and such thatactuation of said switch device produces a constant change in theresonance frequency of said circuit irrespective of the setting of saidganging means.

a variable capacitor connected in parallel with said inductor andproducing equal changes in capacity for equal movements of its movableelement, a. series arrangement of a fixed capacitor and a variablecapacitoralso connected in 128K821? lel with said inductor, meansproviding mechanical ganging between said variable capacitors, and

a switch device connected to short-circuit said fixed capacitor andthereby to changa tha nance frequency of said circuit, saidvariablecapacitor in said series arrangement capacity determining structure:distributed over. the range of movement of its movable element toproduce unequal changes incapacity for equal movements of saidgangingmeansso that operation of said switch device prqduces a constantchange'in the resonant frequency'of thecircult irrespective of thesetting of said ganged condensers.

3. A resonant circuitfcomprising an inductor of inductance L, a variablecapactor of capacitance C1 connected in parallel with said inductor, aseries arrangement of a variable capacitor of capacitance C2 and afixedcapacitor of capacitance C3 also connected; inparallel. with saidinor, me ns providing; mechamcalgan ing be.-

tween said variable capacitomranda: switch device connected toshort-circuit said fixedcanacitor to change the resonance frequency ofsaid circuit, said variable capacitors having capacity determiningstructure distributed over the; range 4 of setting of said mechanical'ganging means substantially to satisfy the formula v where "andF the'irequ'ency change effected by closing said switchdevice, whereby F is asubstantially constanttirequeney change irrespective of the settingof-said ganging means.

EESHE HERBERT BEDFORD.

HENRY ASHFORD WYNN.

REFERENCES CITED The following references, are. of record in the file 0ipatent Country Date Great Britain Aug. 8 1938 OTHER aEFERENcEs f'Ihe'Wireless Wo'rldP November I7, 1938', title,

Number Band Spread-ing G; 05 F; Whitaliter;

